The processing of a microwave-frequency wave, for example received by a satellite, requires the development of specific components, allowing propagation, amplification, and filtering of this wave.
For example a microwave-frequency wave received by a satellite must be amplified before being returned to the ground. This amplification is possible only by separating the set of frequencies received into channels, each corresponding to a given frequency band. Amplification is then carried out channel by channel. The separation of the channels requires the development of bandpass filters.
The development of satellites and the increased complexity of the signal processing to be performed, for example reconfiguration of the channels in flight, has led to the necessity to implement frequency-tunable bandpass filters, that is to say filters for which it is possible to adjust the central filtering frequency customarily referred to as the filter tuning frequency.
One of the known technologies of bandpass filters tunable in the microwave region is the use of passive semi-conducting components, such as PIN diodes, continuously variable capacitors or capacitive switches. Another technology is the use of MEMS (for micro electromechanical systems) of ohmic or capacitive type.
These technologies are complex, inefficient in terms of electrical energy and not very reliable. These solutions are also limited at the level of the signal power processed. Moreover a consequence of frequency tunability is an appreciable degradation in the performance of the filter, such as its quality factor Q. Finally, the RF losses (band achieved, “Return Loss”, insertion losses etc. . . . ) are degraded by the change of frequency.
Furthermore, the technology of filters based on dielectric elements is known in the art. The use of dielectric elements makes it possible to produce non-tunable bandpass filters.
These filters typically comprise an at least partially closed cavity, comprising a conducting wall (typically metallic for example made of aluminium or INVAR™, or other similar alloys) in which is disposed a dielectric element, typically of round or square shape (the dielectric material is typically zirconia, alumina or barium magnesium tantalate (BMT)).
An input excitation means introduces the wave into the cavity (for example a coaxial cable terminated by an electrical probe or a waveguide coupled by an iris) and an output excitation means of like nature makes it possible for the cavity to output the wave.
A bandpass filter allows the propagation of a wave over a certain frequency span and attenuates this wave for the other frequencies. A passband and a central frequency of the filter are thus defined. For frequencies around its central frequency, a bandpass filter has high transmission and low reflection.
The passband of the filter is characterized in various ways according to the nature of the filter.
The parameter S is a parameter which expresses the performance of the filter in terms of reflection and transmission. S11, or S22, corresponds to a measure of reflection and S12, or S21, to a measure of transmission.
A filter carries out a filtering function. This filtering function can generally be approximated via mathematical models (Chebychev functions, Bessel functions, etc.). These filtering functions are generally based on ratios of polynomials.
For a filter carrying out a filtering function of Chebychev or generalized Chebychev type, the passband of the filter is determined at equi-ripple of S11 (or S22), for example at 15 dB or 20 dB reduction in reflection with respect to its out-of-band level. For a filter carrying out a function of Bessel type, the band is taken at −3 dB (when S21 crosses S11 if the filter has negligible losses).
A filter typically comprises at least one resonator comprising the metallic cavity and the dielectric element. A mode of resonance of the filter corresponds to a particular distribution of the electromagnetic field which is excited at a particular frequency.
In order to increase filter selectivity, that is to say the capacity of the filter to attenuate the signal outside of the passband, these filters can be composed of a plurality of mutually coupled resonators.
The central frequency and the passband of the filter depend both on the geometry of the cavities and dielectric elements, as well as the mutual coupling of the resonators as well as couplings with the filter input and output excitation means. Coupling means are for example openings or slots referred to as irises, electrical or magnetic probes or microwave lines.
The filter allows through a signal whose frequency lies in the passband, but the signal is nonetheless attenuated by the filter losses.
The tuning of the filter making it possible to obtain a transmission maximum for a given frequency band is very challenging and depends on the whole set of parameters of the filter. It is moreover dependent on the temperature.
In order to perform an adjustment of the filter so as to obtain a precise central frequency of the filter, the resonant frequencies of the resonators of the filter can be very slightly modified with the aid of metallic screws, but this method performed in an empirical manner is very time consuming and allows limited frequency tunability, typically of the order of a few %. In this case, the objective is not tunability but the obtaining of a precise value of the central frequency, and it is desired to obtain reduced sensitivity of the frequency of each resonator in relation to the depth of the screw.
The circular or square symmetry of the resonators simplifies the design of the filter.
Depending on its geometry, generally a resonator has one or more resonant modes each characterized by a particular (distinctive) distribution of the electromagnetic field giving rise to a resonance of the microwave-frequency wave in the structure at a particular frequency. For example, TE (for Transverse Electric or H) or TM (for Transverse Magnetic or E) modes of resonance having a certain numbers of energy maxima labelled by indices, may be excited in the resonator at various frequencies. FIG. 1 illustrates, by way of example, the resonant frequencies (f) of the various modes for an empty circular cavity as a function of the dimensions of the cavity (diameter D and height H). FIG. 1 illustrates the square of the resonance frequency f multiplied by the diameter D divided by 104, (f.D/104)2 as a function of the square of the diameter D of the cavity divided by the height H of the cavity, (D/H)2 for different modes TE and TM defined by the numbers of maxima labelled by three subscripts, for example, TE111, TE011, TE212, TM110, and TM011, etc.
To optimize the compactness of the filters, resonator filters operating on several modes (typically 2 or 3) are known in the art. In particular, filters operating according to a dual mode (“dual mode filter”) are known. These modes have two perpendicular polarizations Px and Py having a distinctive and specific distribution of the electromagnetic field in the cavity: the distributions of the electromagnetic fields corresponding to the two polarizations are orthogonal and the distributions corresponding to the two polarizations Px and Py are deduced or obtained from one another by a rotation of 90° about an axis of symmetry of the resonator.
If the symmetry of the resonator is perfect, the two orthogonal polarizations possess the same resonant frequency and are not coupled. Coupling between polarizations is obtained by breaking the symmetry, for example by introducing a discontinuity (perturbation) at 45° to the polarization axes Px and Py, typically with the aid of metallic screws.
Moreover, the resonant frequencies can be tuned (optionally to different frequencies) by introducing discontinuities (perturbations) into the polarization axes Px and Py.
Thus the two polarizations Px and Py of a dual mode can resonate according to one and the same frequency (symmetry in relation to the polarization axes) or according to two slightly different frequencies (dissymmetry in relation to the polarization axes).
The dual modes thus make it possible to produce two electromagnetic resonances in a single resonant element. Several modes possessing these particular field distributions can be used. For example the dual modes TE11n (H11n) are extensively used in cavity filters since they culminate in a good compromise between a large quality factor (the compromise being more with an increasing value of the index n, n being an integer), reduced bulk (reduced by a factor of about 2 by employing dual modes) and significant frequency isolation with respect to the other resonant modes (that one does not desire to couple to ensure proper operation of the filter).